1. Field of the Invention
The invention relates to compositions for coating glass surfaces and, more specifically, coating compositions for optical waveguides such as optical fibers, the coating compositions having improved mechanical characteristics and adhesion to the glass.
2. Description of the Related Prior Art
Optical fiber waveguides have been coated with plastic compositions of various materials in order to protect the fiber and increase its tensile strength. The coating materials that have been utilized include polyvinylidene fluoride, polymethylsiloxane, polyesters, polyimides, polyester imides, epoxides and polyurethanes.
For example, U.S. Pat. No. 4,099,837 discloses a coating composition for an optical fiber waveguide, the coating composition is a prepolymer made by mixing certain epoxy resins and reacting the mixture with acrylic and/or methacrylic acid. The mixture contains an aliphatic-type glycidyl ether and an aromatic-type glycidyl ether, such as a mixture of 1,4-butanediol diglycidyl ether and a diglycidyl ether of bisphenol A or substituted, e.g., halogenated, bisphenol A, such as brominated bisphenol A. The proportion of aliphatic diglycidyl ether to aromatic diglycidyl ether is 0.4 to 1.0 on a weight ratio basis. The epoxy resin mixture is reacted with acrylic and/or methacrylic acid on an equivalent basis with between 0.5 to 1.0 mol of acid per epoxy equivalent weight, and preferably 0.8-0.95 mol of acid per equivalent weight.
U.S. Pat. No. 4,311,726 discloses a method for manufacturing an optical waveguide with a plastic layer of the epoxy-acrylate type located on the lightguide fiber. A radiation-hardenable prepolymer composition is applied to the lightguide fiber immediately after the fiber-drawing process. The prepolymer is hardened by actinic radiation and includes an acylated hydroxy ester of acrylic and/or methacrylic acid and an epoxide.
U.S. Pat. No. 5,229,433 discloses a liquid, radiation-curable coating composition for the coating of glass surfaces. The coating composition includes 56 to 89% by weight of one or more diethylenically unsaturated polyurethanes optionally containing urea groups, 3 to 30% by weight of one or more ethylenically unsaturated monomers, 0.5 to 8% by weight of one or more photoinitiators and 0.05 to 6% by weight of an alkoxysilane. The ethylenically unsaturated monomers include one or more ethylenically unsaturated monomers containing carboxyl groups, optionally together with other ethylenically unsaturated monomers, and the alkoxysilane contains epoxide groups. The coating compositions are used for the coating of optical glass fibers.
U.S. Pat. No. 5,985,952 discloses a radiation curable primary coating composition for coating an optical fiber composed of a component having a first end and a second end, a sauturated aliphatic backbone based upon a polyethylene/butylene structure, and at least one epoxide group at the first end of the component and at least one reactive functional group at the second end. The composition described also contains a mixture of acrylate monomers, one being monofunctional and one being at least difunctional. The patent also teaches the use of vinyl ether monomers in conjunction with the aliphatic backbone/epoxide moiety to form an IPN.
While the conventional coating compositions have been adequate for most applications, it would be desirable to be able to tailor the properties of a coating formulation for improved stress relaxation properties and still maintain good mechanical characteristics and adhesion to glass.
Accordingly, an object of the invention is to provide a new coating composition for optical fibers, whereby the characteristics of the cured coating can surprisingly be tailored to achieve a desired optical fiber coating specification or specifications.
Another object of the invention is to provide optical fibers coated with the coating composition in accordance with the present application.
These objectives and other objectives are achieved by providing a liquid, radiation curable composition for coating an optical fiber, and a optical fiber coated therewith. The coating composition includes an aliphatic epoxide, a urethane acrylate oligomer, a cationic photoinitiator, a free radical photoinitiator, and a reactive diluent. The cationic photoinitiator is present in the composition in an amount effective to initiate polymerization of the aliphatic epoxide, and the free radical photoinitiator is present in the composition in an amount effective to initiate polymerization of the urethane diacrylate oligomer.
The present invention provides an improved optical fiber coating composition that can be tailored by adjusting the molecular weight and polydispersity of the components of the composition to meet a particular set of optical fiber coating specifications for both primary and secondary coatings. As one skilled in the art would readily recognize, the properties of the coating compositions in accordance with the present invention can also be tailored by controlling the cure conditions, such as temperature, and level of oxygen present in the cure environment.
Compositions in accordance with the present invention include an aliphatic epoxide, a urethane acrylate oligomer, a reactive diluent, and suitable photoinitiators for the aliphatic epoxide and the urethane acrylate oligomer. While not wishing to be bound by theory, it is believed that the composition in accordance with the present invention forms an interpenetrated polymer network. In any event, the photocured composition while relatively hard, has some flexibility between the polymer chains, and can advantageously be tailored to meet the desired characteristic or characteristics of the coating.
Again, while not wishing to be bound by theory, it is believed that the aliphatic epoxide polymerizes under the influence of the cationic photoinitiator. Also, the urethane acrylate polymerizes under the influence of the free radical photoinitiator with some copolymerization with the reactive diluent. However, there is no or substantially no copolymerization between the aliphatic epoxide and the urethane acrylate.
Coating compositions in accordance with the present invention may advantageously be utilized for both primary and secondary coatings for optical fibers. As used herein, the term xe2x80x9cprimary coatingxe2x80x9d is defined as that coating which directly contacts the glass portion of the optical fiber. The primary coating should be liquid at room temperature. The primary coating should have a viscosity suitable for high speed processing, and the primary coating should have a high cure speed. The primary coating should exhibit good adhesion to glass to prevent premature delamination of the coating from the glass portion of the optical fiber. The primary coating should have a low modulus at lower temperatures to minimize the effects of microbend attenuation due to small stresses on the optical fiber itself. The primary coating should have a refractive index high enough to ensure that errant signals escaping from the glass core are refracted back to the core of the optical fiber.
As used herein, the term xe2x80x9csecondary coatingxe2x80x9d is defined as the coating which covers the primary coating on the optical fiber. The secondary coating should have sufficient modulus to give impact resistance and to provide a protective barrier, and give tensile strength to the optical fiber. The secondary coating should exhibit little physical change over a wide temperature range, good resistance to water and solvent absorption and have good color stability. Both primary and secondary coatings should exhibit good characteristics of adhesion, but should still be strippable, such as for field splicing, etc.
When used as primary coatings, cured coatings in accordance with the present invention preferably have a glass transition temperature (Tg) of from about xe2x88x9260 C. to about 0xc2x0 C., more preferably from about xe2x88x9250 to about xe2x88x9230xc2x0 C., and most preferably about xe2x88x9240xc2x0 C., and a low modulus of elasticity of from about 0.5 to about 3.0 MPa, more preferably from about 1.0 to about 2.0 MPa and most preferably about 1.6 MPa. When utilized as a secondary coating, cured coatings in accordance with the present invention preferably have a Tg from about 40 to about 100xc2x0 C., more preferably from about 50 to about 80xc2x0 C., and most preferably about 75xc2x0 C., and a low modulus of elasticity of from about 60 to about 900 Mpa, more preferably from about 500 to about 900 Mpa and most preferably about 700 Mpa.
Coating compositions in accordance with the present invention have a relatively narrow range of molecular weight (unless otherwise noted, all molecular weights expressed herein are weight average molecular weights), preferably ranging from about 100 to about 25,000, more preferably from about 500 to about 20,000, and most preferably about 10,000, when the composition is used as a primary coating. When utilized as a secondary coating, the molecular weight range is preferably from about 100 to about 10,000 more preferably from about 5,000 to about 7,000, and most preferably about 5,000. The polydispersity of the coating compositions in accordance with the present invention preferably ranges from about 1.0 to about 4.0, more preferably from about 1.2 to about 2.5, and most preferably about 2.1. Cured coatings prepared from coating compositions in accordance with the present invention have good adhesion to glass and good stress characteristics.
As used herein, the term xe2x80x9caliphatic epoxidesxe2x80x9d includes, for example, cycloaliphatic epoxides. Suitable aliphatic epoxide resins for purposes of the present invention are those having at least one oxirane functionality; and preferably up to 3 to 4 epoxide functionalities. The molecular weight of the aliphatic epoxide is preferably from about 150 g/mole to about 2,000 g/mole, more preferably from about 150 g/mole to about 1500 g/mole and most preferably about 500 g/mole. The epoxide may exhibit other functionalities such as hydroxyl and or vinyl unsaturation within the molecule. For example, the aliphatic epoxide may be a cycloaliphatic epoxide including a 3,4-epoxycyclohexylmethyl-3,4-epoxycyclohexane carboxylate having the general formula (I): 
wherein R1 through R18 which can be the same or different, are hydrogen or alkyl radicals generally containing one to nine carbon atoms inclusive, and preferably containing one to three carbon atoms inclusive, as for example methyl, ethyl, n-propyl, n-butyl, n-hexyl, 2-ethylhexyl, n-octyl, n-nonyl and the like. Preferably, R1 through R18 are each hydrogen.
Cycloaliphatic epoxide compounds within the scope of the above formula that may be used in the present invention include the following nonexhaustive list: 3,4-epoxycyclohexylmethyl-3,4-epoxycyclohexane carboxylate; 3,4-epoxy-1-methylcyclohexyl methyl-3,4-epoxy-1-methylcyclohexane carboxylate; 6-methyl-3,4-epoxycyclohexymethyl-6-methyl-3,4-epoxycyclohexane carboxylate; 3,4-epoxy-3-methylcyclohexylmethyl-3,4-epoxy-3-methylcyclohexane carboxylate; and 3,4-epoxy-5-methylcyclohexylmethyl-3,4-epoxy-5-methylcyclohexane carboxylate. Cycloaliphatic epoxide resins suitable for use in the present invention are available from UCB Chemical Corp. of Smyrna, Ga. They are sold under the name Uvacure(copyright) and include Uvacure(copyright) 1500, 1530, 1531, 1532, 1533 and 1562. Other cycloaliphatic epoxide compounds suitable for use in the present invention will be apparent to one skilled in the art. In this regard, other suitable cycloaliphatic epoxide compounds are disclosed in U.S. Pat. No. 2,890,194, the disclosure of which is incorporated herein by reference.
Of course, other aliphatic epoxides may be utilized in the present invention including, but not limited to glycidyl ethers or esters having the following general formulae (II) and (III), respectively: 
wherein R in both formulae (II) and (III) represents, for example, an acrylate group such as 1,2-epoxy propyl acrylate or 1,2-epoxy ethyl acrylate, a silane group such as an amino-functional silane, an acrylamido-functional silane, a mercapto-functional silane, an allyl-functional silane, a vinyl-functional silane, a methylacrylate-functional silane, and an acrylate-functional silane, or an aryl group, such as butyl, cresyl, phenyl, nonyl phenyl, p-tert-butyl phenyl and 2-ethyl hexyl. Preferably, the silane group is a mercaptoalkyl trialkoxy silane, a methacryloyxlalkyl trialkoxy silane, an aminoalkyl trialkoxyl silane, a vinyl trialkoxyl silane and mixtures thereof. More preferably, the silane is 3-aminopropyl triethoxy silane, 3-methacryloxy-propyltrimethoxy silane, gamma-mercaptopropyl trimethoxy silane, gama-mercaptopropyl (gamma-mercaptopropyl)triethoxy silane, beta-(3,4-epoxycyclohexyl)ethyl trimethoxy silane, gamma-glycidoxypropyl trimethoxy silane, 3-vinylthiopropyl trimethoxy silane, vinyl-tris-(beta-methoxyethoxy)silane, or vinyl triacetoxy silane.
For example, the aliphatic epoxide may be a glycidyl ether epoxy resin prepared by reacting bisphenol A with epichlorohydrin, the resin having the following general (IV): 
wherein n can be from 1 to about 30. It will be understood that the bisphenol A used to prepare the epoxide may be hydrogenated bisphenol A in which case the resulting epoxy resins are analogous to those of formula IV above but with hydrogenated rings.
The aliphatic epoxide may also be a glycidyl ether epoxy resin prepared by reacting bisphenol A with an alcohol such as n-butyl alcohol and epichlorohydrin, this resin having the following general formula (V): 
Suitable aliphatic epoxides for use in the present invention may also be prepared by reaction of novolac phenolic resins including, for example, o- or p-cresol formaldehyde novolacs, and epichlorohydrin, this resin having the following general formula (VI): 
wherein n can be from 1 to about 50.
One skilled in the art will also recognize that a variety of other aliphatic epoxy products may be utilized in the present invention, including but not limited to epoxidized soy and linseed oils, as well as epoxy products made by the reaction of polyols including, for example, glycerol, sorbitol, and polyethylene or polypropylene glycols, with epichlorohydrin and a base catalyst, as is conventionally known. For example, the aliphatic epoxy may be a polyethylene glycol diglycidyl ether of the following general formula (VII): 
wherein n can be from 1 to about 40.
Also useful in the present invention are low molecular weight cycloaliphatic diepoxy compounds prepared by epoxidation of the corresponding alkenes, generally with peracetic acid. These compounds include, for example, 3,4-epoxycyclohexylmethyl 3xe2x80x2,4xe2x80x2-epoxy-4-cyclohexylcarboxylate of formula (VIII) below and diglycidyl 1,2-cyclohexane dicarboxylate of formula (IX) below: 
Other suitable aliphatic epoxides will be apparent to one skilled in the art. Also, mixtures of aliphatic epoxides may be used in the present invention.
The urethane acrylate oligomer of the present invention may be formed by reacting a polyol, for example a diol, with a multifunctional isocyanate, for example, a disocyanate, and then end-capping with a hydroxy-functional acrylate.
The polyol may be a polyether polyol, a polyester polyol, a polycarbonate polyol, a hydrocarbon polyol, a polybutadiene polyol, copolymers of the previously listed polyols, and mixtures of the foregoing.
The polyol may be a polyol with a number average molecular weight of about 200-10,000.
The polyether polyols may be homopolymers or copolymers of alkylene oxides including C2 to C5 alkylene oxides such as, for example, ethylene oxide, propylene oxide, butylene oxide, tetrahydrofuran, and 3-methyltetrahydrofuran; homopolymers or copolymers of the above alkylene oxides obtained by using, as an initiator, C14 to C40 polyols, such as 12-hydroxystearyl alcohol and hydrogenated dimerdiol; and adducts of the above alkylene oxides with bisphenol-A or hydrogenated bisphenol-A. These polyether polyols may be used alone or in combination of two or more. Other suitable polyether polyols will be apparent to one skilled in the art.
The polyester polyols may be, for example, addition reaction products of a diol component and a lactone, reaction products of the diol component and a polyvalent carboxylic acid, and addition reaction products of three components, including the diol component, a dibasic acid, and the lactone. The diol component may be C2 to C40 aliphatic diols with a low molecular weight such as ethylene glycol, propylene glycol, diethylene glycol, dipropylene glycol, 1,4-butanediol, 1,5-pentanediol, 3-methyl-1,5-pentanediol, 2,4-diethyl-1,5-pentanediol, 1,6-hexane glycol, neopentyl glycol, 1,9-nonanediol, 1,10-decanediol, 12-hydroxystearyl alcohol, and hydrogenated dimerdiol; and an alkylene oxide adduct of bisphenol-A. The lactone may be, for example, epsilon-caprolactone, delta-valerolactone, and beta-methyl-delta-valerolactone. The polyvalent carboxylic acid may be, for example, aliphatic dicarboxylic acids such as succinic acid, adipic acid, azelaic acid, sebacic acid, and dodecanedioic acid; and aromatic dicarboxylic acids such as hexahydrophthalic acid, tetrahydrophthalic acid, phthalic acid, isophthalic acid, and terephthalic acid. Other suitable polyester polyols will be apparent to one skilled in the art.
The polycarbonate polyols may be, for example, polycarbonate diols which are obtainable by a reaction of a short chain dialkylcarbonate and a component selected from aforementioned polyether polyols, polyester polyols and diol components such as 2-methylpropanediol, dipropylene glycol, 1,4-butanediol, 1,6-hexanediol, 3-methyl-1,5-pentanediol, neopentyl glycol, 1,5-octanediol, and 1,4-bis-(hydroxymethyl)cyclohexane. The short chain dialkylcarbonate may be C1 C4 alkylcarbonates such as, for example, dimethylcarbonate and diethylcarbonate. Other suitable polycarbonate polyols will be apparent to one skilled in the art.
If necessary, polyols with a relatively low molecular weight may be used. Examples of polyols with a relatively low molecular weight include ethylene glycol, propylene glycol, tripropylene glycol, 1,3- or 1,4-butanediol, neopentyl glycol, 1,6-hexanediol, 1,9-nonanediol, 1,10-decanediol, higher fatty acid polyols and higher hydrocarbon polyols such as castor oil, coconut oil, monomyristins (1-monomyristin and 2-monomyristin), monopalmitins (1-monopalmitin and 2-monopalmitin), monostearins (1-monostearin and 2-monostearin), monooleins (1-monoolein and 2-monoolein), 9,10-dioxystearic acid, 12-hydroxyricinoleyl alcohol, 12-hydroxystearyl alcohol, 1,16-hexadecanediol (juniperic acid or a reduction product of thapcic acid), 1,21-henicosanediol (a reduction product of Japanese acid), chimyl alcohol, batyl alcohol, selachyl alcohol, and dimeric acid diol. Other suitable polyols with a relatively low molecular weight will be apparent to one skilled in the art.
The polybutadiene polyol may be, for example, hydroxyl-terminated, fully or partially hydrogenated 1,2-polybutadiene or 1,2-polybutadiene polyol hydrogenated to an iodine number of from 9 to 21.
The multi-functional isocyanate used in the present invention may be, for example, an aromatic polyisocyanate, an aromatic aliphatic polyisocyanate, an alicyclic polyisocyanate, and an aliphatic polyisocyanate.
Examples of the aromatic polyisocyanate include diisocyanates such as m-phenylene diisocyanate, p-phenylene diisocyanate, 4,4xe2x80x2-diphenyl diisocyanate, 1,5-naphthalene diisocyanate, 4,4xe2x80x2-diphenylmethane diisocyanate, 2,4- or 2,6-tolylene diisocyanate, 4,4xe2x80x2-toluidine diisocyanate, and 4,4xe2x80x2-diphenyl ether diisocyanate; and polyisocyanates such as triphenylmethane-4,4xe2x80x2,4xe2x80x3-triisocyanate, 1,3,5-triisocyanatebenzene, 2,4,6-triisocyanatetoluene, and 4,4xe2x80x2-diphenylmethane-2,2xe2x80x2,5,5xe2x80x2-tetraisocyanate.
Examples of the aromatic aliphatic polyisocyanate include diisocyanates such as 1,3- or 1,4-xylylene diisocyanate or mixtures thereof and 1,3- or 1,4-bis(1-isocyanate-1-methylethyl)benzene or mixtures thereof; and polyisocyanates such as 1,3,5-triisocyanatemethylbenzene.
Examples of the alicyclic polyisocyanate include diisocyanates such as 1,3-cyclopentene diisocyanate, 1,4-cyclohexane diisocyanate, 1,3-cyclohexane diisocyanate, 3-isocyanatemethyl-3,5,5-trimethylcyclohexyl isocyanate (isophorone diisocyanate), 4,4xe2x80x2-methylenebis(cyclohexyl isocyanate), methyl-2,4-cyclohexane diisocyanate, methyl-2,6-cyclohexane diisocyanate, and 1,3- or 1,4-bis(isocyanatemethyl)cyclohexane; and polyisocyanates such as 1,3,5-triisocyanatecyclohexane, 1,3,5-trimethylisocyanatecyclohexane, 2-(3-isocyanatepropyl)-2,5-di(isocyanatemethyl)-bicyclo(2.2.1)heptane, 2-(3-isocyanatepropyl)-2,6-di(isocyanatemethyl)-bicyclo(2.2.1)heptane, 3-(3-isocyanatepropyl)-2,5-di(isocyanatemethyl)-bicyclo(2.2.1)heptane, 5-(2-isocyanateethyl)-2-isocyanatemethyl-3-(3-isocyanatepropyl)-bicyclo(2.2.1)heptane, 6-(2-isocyanateethyl)-2-isocyanatemethyl-3-(3-isocyanatepropyl)-bicyclo(2.2.1)heptane, 5-(2-isocyanateethyl)-2-isocyanatemethyl-2-(3-isocyanatepropyl)-bicyclo(2.2.1)heptane, and 6-(2-isocyanateethyl)-2-isocyanatemethyl)-2-(3-isocyanatepropyl)-bicyclo(2.2.1)heptane.
Examples of the aliphatic polyisocyanate include diisocyanates such as trimethylene diisocyatnate, tetramethylene diisocyanate, hexamethylene diisocyanate, pentamethylene diisocyanate, 1,2-propylene diisocyanate, 1,2-butylene diisocyanate, 2,3-butylene diisocyanate, 1,3-butylene diisocyanate, 2,4,4- or 2,2,4-trimethylhexamethylene diisocyanate, and 2,6-diisocyanatemethylcaproate; and polyisocyanates such as lysine ester triisocyanate, 1,4,8-triisocyanateoctane, 1,6,11-triisocyanateundecane, 1,8-diisocyanate-4-isocyanatemethyloctane, 1,3,6-triisocyanatehexane, and 2,5,7-trimethyl-1,8-isocyanate-5-isocyanatemethyloctane.
Moreover, derivatives from the above polyisocyanates can be used. Examples of the derivatives include a dimer, a trimer, biuret, allophanate, carbodiimide, polymethylenepolyphenyl polyisocyanate (referred to as crude MDI or polymeric MDI), crude TDI, and an adduct of an isocyanate compound and a polyol with a low molecular weight.
The hydroxy-functional acrylate may be, for example, 2-hydroxyethyl(meth)acrylate, 2-hydroxypropyl(meth)acrylate, 2-hydroxybutyl(meth)acrylate, 4-hydroxybutyl(meth)acrylate, pentanediol mono(meth)acrylate, 2-hydroxy-3-phenyloxypropyl(meth)acrylate, 2-hydroxyalkyl(meth)acryloyl phosphate, 4-hydroxycyclohexyl(meth)acrylate, cyclohexanedimethanol mono(meth)acrylate, neopentyl glycol mono(meth)acrylate, trimethylolpropane di(meth)acrylate, and pentaerythritol tri(meth)acrylate. Additional examples include compounds which are obtainable by an addition reaction of a glycidyl group-containing compound and a (meth)acrylic acid, such as alkyl glycidyl ether and glycidyl(meth)acrylate. The above hydroxyl group-containing (meth)acrylates may be used alone or in combination of two or more.
Suitable multifunctional isocyanates are toulene diisocyanate (TDI), isophorone diisocyanate (IPDI), and 4,4xe2x80x2 methylene dicyclohexyl diisocyanate (Desmophen W., Bayer Co., USA). Suitable unsaturated monomers containing a hydroxyl group or groups are hydroxyethylacrylate (HEA) and hydroxyethylmethacrylate (HEMA). Suitable polyols are 1,6 hexanediol (Bayer Co. USA), PTG-L 1000 (polyether)(Hodogaya Chemical Co. of Japan), Duracarb(trademark) 122 (Polycarbonate)(PPG Ind, USA), polycaprolactones such as the Tone(trademark) polyester series manufactured by Union Carbide Co. USA, and polybutadiene polyols available from Shell Co. USA as Kraton(trademark).
The molecular weight range of the urethane acrylate oligomer may vary from 500 to 20,000, preferably from 5,000 to 20,000 and more preferably from 5,000 to 15,000, based upon the specific requirements for properties of the primary and/or secondary coating in accordance with the present invention.
The urethane acrylate oligomer resin useful in accordance with the present invention may have the following general formula (X):
R1xe2x80x94(I1xe2x80x94Pxe2x80x94)nxe2x80x94I2xe2x80x94R2xe2x80x83xe2x80x83(X)
wherein
R1=independently, radiation curable functionality, such as hydroxyl ethyl acrylate or hydroxyl ethyl methacrylate
R2=independently, radiation curable functionality such as hydroxyl ethyl acrylate or hydroxyl ethyl methacrylate
I1,2=isocyanate such as IPDI, Desmophen W, TDI
P=polyether polyol, polyester polyol, polycarbonate polyol, hydrocarbon polyol, polybutadiene polyol, co-polymer polyols of the same and mixtures of the same. (Depends on polyol selected)
Suitable polyester diol, polyether, polybutadiene and copolymer diol materials for use in the present invention include: 
where n=1 to 40
Polyester diol 
where n=1 to 40
Polyether diol 
Polybutadiene diol 
where m and n each are 1 to 40
Copolymer diol
Suitable polyether-based, aliphatic urethane diacrylate compounds for use in the present invention are available from UCB Chemical Corp., of Smynra, Ga. They are sold under the name Ebercryl and include Ebecryl 230. Other polyether-based, aliphatic urethane diacrylate compounds suitable for use in the present invention will be apparent to one skilled in the art. In this regard, other suitable polyether-based, aliphatic urethane diacrylate compounds are disclosed in U.S. Pat. Nos. 4,284,731, 4,624,994, 4,607,084, 4,525,258, the disclosures of which are all incorporated herein by reference. Polyester-based, aliphatic urthane acrylate oligomers are available from Sartomer of Exton, Pa. or from Henkel. They are sold under the name CN966XXX, including CN966J75, and Photomer 6010, respectively.
In general, the aliphatic epoxide and the urethane diacrylate oligomer may be used in a weight ratio of aliphatic epoxide to urethane acrylate oligomer of from about 1:9 to about 9:1, more preferably about 1:3 to about 3:1, and most preferably about 1:1.
In accordance with the present invention, a cationic-type photoinitiator is used to initiate polymerization of the aliphatic epoxide, and a free radical-type photoinitiator is used to separately initiate the polymerization of the urethane acrylate.
Suitable cationic-type photoinitiators include, for example, aromatic onium salts, such as iodonium, sulfonium, arsonium, azonium, bromonium and selenonium salts. The onium salts are preferably chemically modified to render them more hydrophobic, for example, by incorporating saturated hydrocarbon moieties such as alkyl or alkoxy substituents of from about 4 to about 18 carbon atoms. Illustrative cationic photoinitiators that may be used in the present invention include the following: (4-octyloxyphenyl)phenyl iodonium hexafluoro antimonate; (4-octyloxyphenyl)diphenyl sulfonium hexafluoro antimonate; (4-decyloxyphenyl)phenyl iodonium hexafluoro antimonate; and (4-octadecyloxyphenyl)phenyl iodonium hexafluoro antimonate. Cationic photoinitiators suitable for use in the present invention are available from the Sartomer Company of Exton, Pa. They are sold under the name SARCAT(trademark). Other suitable cationic photoinitiators will be apparent to one skilled in the art.
The cationic-type photoinitiator in accordance with the present invention may be used in an amount of about 0.25 to about 10% by weight, preferably about 0.25 to about 5% by weight, and most preferably about 1% by weight, based upon the total weight of the composition.
Suitable free radical-type photoinitiators include, for example, an acyl phosphine oxide photoinitiator, more specifically, a benzoyl diaryl phosphine oxide photoinitiator. Examples of suitable benzoyl diaryl phosphine oxide photoinitiators include: 2,4,6-trimethylbenzoyldiphenyl-phosphine oxide (Lucerin TPO available from BASF); and bis(2,6-dimethoxybenzoyl)-2,4,4-trimethylpentylphosphine oxide (Irgacure 1700 available from Ciba Additives, Tarrytown, N.Y.). Examples of other free radical-type photoinitiators that may be used in the present invention include:
hydroxycyclohexylphenylketone;
hydroxymethylphenylpropanone;
dimethoxyphenylacetophenone;
2-methyl-1-[4-(methyl thio)-phenyl]-2-morpholinopropanone-1;
1-(4-isopropylphenyl)-2-hydroxy-2-methylpropan-1-one;
1-(4-dodecyl-phenyl)-2-hydroxy-2-methylpropan-1-one;
4-(2-hydroxyethoxy)phenyl-2(2-hydroxy-2-propyl)-ketone;
diethoxyphenyl acetophenone;
2,4,6 trimethylbenzoyl diphenylphosphone;
a mixture of (2,6-dimethoxy benzoyl)-2,4,4 trimethylpentylphosphineoxide and 2-hydroxy-2-methyl-1-phenyl-propane-1-one; and mixtures of the foregoing.
Preferably, the free radical photoinitiator is a mixture of phosphine oxide photoinitiators an example of which is Darocur 4265. Free radical-type photoinitiators suitable for use in the present invention are available from Ciba Additives of Tarrytown, N.Y. They are sold under the names Irgacure(copyright) and Darocur(copyright). Other suitable free radical-type photoinitiators will be apparent to one skilled in the art.
The free radical-type photoinitiator in accordance with the present invention may be used in an amount of about 0.25 to about 10% by weight, more preferably about 0.25 to about 5% by weight, and most preferably about 1% by weight based upon the total weight of the composition.
In accordance with the present invention, a reactive diluent or mixture of reactive diluents is included in the coating composition. While not wishing to be bound by theory, it is believed that, during the polymerization of the urethane acrylate oligomer, the reactive diluent hooks together chains of the urethane acrylate oligomer. In any event, adequate curing of the coating compositions of the present invention is ensured by the presence of the reactive diluent as it functions as a solvent improving the solubility of the aliphatic epoxide and the urethane acrylate oligomer in solution. The use of the reactive diluent also allows the formulator to adjust the viscosity of the solution to improve processability. The reactive diluent may, for example, be a lower molecular weight, liquid acrylate-functional compound include the following diacrylates and monofunctional acrylates.
1,6-hexanediol diacrylate, 
1,4-butanediol diacrylate, 
ethylene glycol diacrylate, 
diethylene glycol diacrylate, 
tetraethylene glycol diacrylate, 
tripropylene glycol diacrylate, 
neopentyl glycol diacrylate, 
1,4-butanediol dimethacrylate, 
poly(butanediol)diacrylate, 
tetrathylene glycol dimethacrylate, 
1,3-butylene glycol diacrylate, 
tetraethylene glycol diacrylate, 
triisopropylene glycol diacrylate, 
triisopropylene glycol diacrylate, 
ethyoxylated bisphenol A diacrylate, 
isoborynl acrylate 
n-vinyl caprolactam 
Preferably, lower molecular weight, liquid monofunctional acrylate compounds suitable for use as a reactive diluent(s) in accordance with the present invention are isobornyl acrylate and n-vinyl caprolactam. Other reactive diluents suitable for us in the present invention will be apparent to one skilled in the art.
The reactive diluent may be used in the coating composition in accordance with the present invention in an amount of from about 5 to about 80% by weight, preferably from about 5 to about 60% by weight, and most preferably about 30% by weight, based upon the total weight of the composition.
The coating compositions in accordance with the present invention preferably include an adhesion promoter. Suitable adhesion promoters for purposes of the present invention include acid functional materials and organofunctional silanes. For example, the organofunctional silane may be an amino-functional silane, an acrylamido-functional silane, a mercapto-functional silane, an allyl-functional silane, a vinyl-functional silane, a methylacrylate-functional silane, and an acrylate-functional silane. Preferably, the organofunctional silane is mercaptoalkyl trialkoxy silane, a methacryloyxlalkyl trialkoxy silane, an aminoalkyl trialkoxyl silane, a vinyl trialkoxyl silane and mixtures thereof. More preferably, the organofunctional silane is 3-aminopropyl triethoxy silane, 3-methacryloxy-propyltrimethoxy silane, gamma-mercaptopropyl trimethoxy silane, gama-mercaptopropyl(gamma-mercaptopropyl)triethoxy silane, beta-(3,4-epoxycyclohexyl)ethyl trimethoxy silane, gamma-glycidoxypropyl trimethoxy silane, 3-vinylthiopropyl trimethoxy silane, vinyl-tris-(beta-methoxyethoxy)silane, vinyl triacetoxy silane, and mixtures thereof. A particularly preferred adhesion promoter is KBM 803 availiable from Shin-Etsu Silicones of America, Inc, of Torrance, Calif.
The adhesion promoter, if used, may be present in the coating composition in accordance with the present invention in an amount of from about 0.1 to about 10% by weight, more preferably from about 0.1 to about 3% by weight, and most preferably less than about 1% by weight, based upon the total weight of the composition.
Other components that may be utilized in the coating composition in accordance with the present invention include antioxidants, flow control agents, sensitizers, stabilizers, lubricants and wetting agents, as is conventional in the art.
The present formulation may include photosensitizers which absorb UV or visible light and transfer energy to the photoinitiators which induces additional decomposition of the photoinitiators with subsequent free radical formation. Examples of effective photosensitizers are thioxanthen-9-one, 2-isopropylthioxanthen-9-one and 10-phenylphenoxazine. The photosensitizer, if used, may be present in the coating composition in accordance with the present invention in an amount of from about 0.25 to about 10% by weight, more preferably about 0.25 to about 5% by weight, and most preferably about 1% by weight based upon the total weight of the composition.
In accordance with the present invention, the foregoing components are mixed or blended together using conventional equipment and an optical fiber may be coated with the coating composition in accordance with the present invention by any conventional optical fiber production technique. The conventional techniques usually involve a draw tower in which a preformed glass rod is heated to produce a thin fiber of glass. The fiber is pulled vertically through the draw tower and, along the way, the fiber passes through one or more coating stations at which various coatings are applied and cured in-line to the newly drawn fiber. The coating stations may each contain a die having an exit orifice that is sized to apply the particular coating to the fiber in a desired thickness. Monitoring and measuring devices may be provided near each station to ensure that the coating applied at that station is coated concentrically and to the desired diameter. Optical fiber coating techniques that may be utilized in the present invention are disclosed in U.S. Pat. Nos. 4,374,161; 5,858,053; 4,962,992; 5,104,433, the disclosures of which are incorporated herein by reference. Other suitable optical fiber coating techniques that may be utilized in the present invention would be apparent to one skilled in the art.